Cardiac arrhythmias account for more than 300,000 sudden deaths each year in the U.S. alone. Our laboratory is investigating the pathogenesis of cardiac arrhythmias. We focus on two arrhythmic disorders: long-QT syndrome (LQT) and idiopathic ventricular fibrillation (IVF), both of which cause sudden death in the young, otherwise healthy, individuals. During the past 8 years of this project, we focused on genetics and in vitro electrophysiology of LQT and IVF. Together with other scientists, we have defined a genetic pathway for pathogenesis of both LQT and IVF. Further exploration of pathogenic mechanisms of LQT and IVF at the tissue and organ level is impossible because of lack of fresh heart tissues from patients. In the proposed studies we plan to develop and characterize LQT- and IVF-animal models in which SCN5A (the cardiac sodium channel gene) mutations are engineered into the mouse genome to further explore the etiology of arrhythmogenesis. We have successfully established a mouse model for LQT and ventricular arrhythmias by targeting an SCN5A mutation (N1325S). Characterization of our arrhythmic mice has led to the working hypothesis that early and after depolarizations (EADs and DADs) are the substrate for ventricular tachycardia (VT) and ventricular fibrillation (VF). In the proposed studies we plan to continue to study the mouse model for LQT to uncover detailed molecular mechanisms of cardiac arrhythmias, and to generate and characterize mouse models for IVF and acquired LQT. Our specific aims are: (1) To investigate whether over-expression of an LQT-causing mutation of SCN5A in the mouse heart will trigger electrophysiological remodeling; (2) To systematically dissect EADs and DADs induced by a genetic LQT mutation; (3) To systematically determine the effects of representative agents from each class of antiarrhythmic drugs on VT/VF and correlate the findings with results on EADs/DADs; (4) To characterize SCN5A mutations associated with IVF and acquired LQT using the transgenic mouse technology. The successful accomplishment of goals in this proposal will provide a fundamental understanding of the pathogenic mechanisms of cardiac arrhythmias. Evaluation of animal models will help define the physiological and cellular processes involved in arrhythmogenesis, and bridge the gap between the in vitro biophysical defects and the in vivo whole animal phenotype characterized by arrhythmia susceptibility. These studies may provide a new framework for the rational design of therapeutic agents.